The present invention relates to a seat belt system that is provided on a vehicle, for example, an automobile, for the purpose of protecting an occupant. More particularly, the present invention relates to a seat belt retractor designed so that a reel shaft for winding up a seat belt is locked from rotating when an emergency situation occurs, thereby preventing the seat belt from being pulled out.
A typical conventional seat belt retractor mounted on a vehicle, for example, an automobile, includes a reel shaft for winding up a webbing, a frame rotatably supporting both ends of the reel shaft, lock means disposed in between the frame and the reel shaft and arranged to allow rotation of the reel shaft under normal circumstances and to be activated, when need arises, so as to prevent rotation of the reel shaft in at least a direction in which the webbing is unwound, deceleration sensing means (hereinafter referred to as "vehicle sensor") operating when deceleration exceeding a predetermined value acts on the vehicle, and means for activating the lock means in response to the operation of the deceleration sensing means, as disclosed, for example, in the specification and drawings of U.S. Pat. No. 4,796,918.
In such a seat belt retractor, the reel shaft is freely rotatable under normal circumstances. Therefore, the webbing can be unwound freely, and the occupant can move forwardly as desired. When an emergency situation occurs, e.g., when a high degree of deceleration acts on the vehicle, the deceleration sensing means operates on sensing the deceleration, causing the lock activating means to activate the lock means so as to lock the reel shaft from rotating. Consequently, the webbing is prevented from being pulled out, and the occupant's body is restrained by the webbing. Thus, inertial forward movement of the occupant's body is prevented.
The deceleration sensing means in the conventional seat belt retractor of the type described above comprises a casing that is attached to the frame, an actuator that is provided on the casing so as to be pivotable between an inoperative position where it does not actuate the lock activating means and an operative position where it actuates the lock activating means, and an inertia member that is accommodated in the casing and held in an inoperative position under normal circumstances so as to set the actuator in the inoperative position and that operates in an emergency where deceleration exceeding a predetermined value acts on the vehicle so as to set the actuator into the operative position.
The inertia member in the conventional deceleration sensing means is formed in a columnar shape, e.g., a spool-like shape, or a spherical shape. In the case of a columnar inertia member, it is held in an erect position under normal circumstances to set the actuator in the inoperative position. When an emergency situation occurs, the inertia member tilts so as to set the actuator into the operative position. In the case of a spherical inertia member, it is accommodated and held in an inoperative position on the casing under normal circumstances so as to set the actuator in the inoperative position. When an emergency situation occurs, the inertia member rolls forwardly so as to set the actuator into the operative position.
Incidentally, there has recently been an increasing demand for automotive parts to decrease in both size and weight. Accordingly, it has been demanded that the seat beat retractor should also be formed in as lightweight and compact a structure as possible. As a necessary consequence of the reduction in size of the seat belt retractor, it is desirable that the inertia member of the vehicle sensor should also be formed as small as possible.
However, if the inertia member in the prior art is reduced in size, the response to deceleration lowers, whether the columnar inertia member or the spherical inertia member, so that it becomes impossible to detect deceleration reliably. Thus, a reduction in size of the inertia member results in a lowering of reliability. Accordingly, the inertia member in the conventional vehicle sensor cannot be reduced in size satisfactorily. Therefore, it has heretofore been impossible to comply well with the demand for reduction in size and weight of the seat belt retractor.
In many cases, the conventional columnar inertia member is formed such that the upper end surface thereof, which is in contact with the actuator, is recessed. However, if the upper end surface of the inertia member is recessed as in the prior art, the radius of gyration of the inertia member when tilting is relatively small, so that the stroke of the actuator obtained for a given amount of tilting of the inertia member is also small. Therefore, in the conventional deceleration sensing means, the size of the inertia member is enlarged and the arm of the actuator is also lengthened to thereby achieve an increase in the stroke of the actuator. Accordingly, the conventional deceleration sensing means inevitably becomes relatively large and cannot be reduced in size satisfactorily.